Frequency translating circuit



Oct. 31, 1933. E. PETERSON FREQUENCY TRANSLATING CIRCUIT Filed June 20, 1928 FIG. 1

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/NVEN7'0R EUGENE FETERsa/v 5r ATTORNEY l atenteci Get. 31 19 33 STATES FREQUENCY 'TBANSLATING CIRCUIT Eugene Peterson, New York, N. Y., assignor to -Western Electric Company, Incorporated, .New

York, N. Y., a corporation of New York Application June 20, 1928. Serial No. 286,774

4 Claims.

The present application includes subject mate ter previously disclosed in application Serial No. 164,413, filed January 29, 1927, which matured into U. S. Patent bio/1,687,225 on October 9, 1928.

eificiency of frequency translating circuits.

Frequency translating circuits, which are defined for the purpose of this specification as in-- cluding both modulators and demodulators or detectors, may employ electric discharge devices for performing the frequency translation function in which case they may be divided into two main types.

The first type involves input current frequency translation and is more commonly referred to as grid current modulation. In this type the impressed currents are combined in the grid or input circuit. This is accomplished by utilizing the curvature of the grid current characteristic of the discharge device. The second type employs output current frequency translation and is more commonly described as plate current modulation. In this case the impressed currents are combined in the plate or output circuit. Thev combining action is due to the curvature of the plant current characteristic.

When input current modulation or detection is used for producing the useful output products, the discharge device acts as an amplifier for the modulation products produced in the input. circuit. The power output produced by this-method of operation is considerably greater than that obtainable with other known methods. and the operation of the frequency translating circuit is more stable.

A feature of this invention is a circuit arrangement for giving the external input circuit an impedance which is high for the input current modulation or detection products so that the voltages thereof, which may be regarded as generated within the discharge device, are not materially reduced by the internal drop between the input electrodes, and may therefore be efficiently utilized to actuate the device as an amplifier.

Another feature of the invention relates to the use of a reactive impedance arrangement for differentially dividing the potential between the discharge device and the external circuit during opposite half cycles of the impressed Waves to produce input current modulation or detection.

An object of the invention is to improve the 1 In one embodiment, to be hereinafter described, the impedance of the external input circuit of a push-pull modulator is increased for the input current modulation or detectionproducts, by including a choke coil in each of the series arms.

The invention may be more fully understood from the following description in conjunction with the accompanying drawing, in which Figs. 1 and 2 represent different embodiments of the invention, and 1i l igz. 2a shows a modification of a portion of ln Fig. 1, sourcesl and 2 are shown for supplying carrier and modulating waves, respectively, to a space discharge device 3, which together with associated circuits is arranged to effect grid current modulation of the applied waves.

The device 3 contains a cathode 4 heated by current from source or battery 5, which current may be regulated by means of a rheostat 6. The device 3 alsocontains a control electrode or grid 7 and an anode or plate 8. A source or battery 9, is provided for supplying space current to the device.

Band pass filters BPF1 and BPFz are respectively interposed between the sources 1 and 2 and the device 3 to minimize transmission of waves from either source into the other. The filters are shown with their output terminals connected in series with each other and with the primary winding 10 of an input transformer 11, the secondary winding 12 of which is inserted in the input circuit of device 3. 1 Many other suitable means are known for efficiently connecting sources such as 1 and 2 to the input circuit of a device such as 3 and any such means may be used instead of the connection shown. Y J

The input circuit of the device comprises grid 7, secondary winding 12, a reactive impedance element shown as the choke coil 13, and cathode 4.

The output circuit of the device comprises anode 8, cathode 4, source 9 and an output transformer 14 by which the output circuit may be coupled to a load circuit. j

The method of operation of the system of Fig.

1 is similar to that of grid current modulation systems in general as disclosed in application Serial No. 696,766, filed March 4, 1924, which matured into U. S. Patent No. 1,699,711 on January 22, 1929, and also in application Serial No. 164,413, filed January 29, 1927, which matured into U. S. Patent No. 1,687,225 on October 9, 1928,

to which reference may be made for a more detailed description.

Transformer 11 is designed to freely pass waves from sources 1 and 2. At the frequency of the desired grid current modrdation product, however, the impedance looking into secondary 12, with the circuit elements coupled thereto as shown to the left thereof in Fig. 1 may not be sufficiently great to insure efficient amplification of the modulation product. Choke coil 13, which may be Wound upon an iron core, provided for increasing the impedance of the input circuit at the frequency of the modulation product.

It is desirable, however, that the impedance of the choke coil 13 at the frequencies of the.

.equivalent to a small or negligible capacitive reactance.

In case the frequency of the modulation product is relatively close to the frequency of one of the sources of applied waves, it may not be feasible to design choke coil 13, either with air or iron core, to have materially different impedances for the nearly equal frequencies. To provide the required frequency discrimination, it is then necessary to employ a filter or network in place of the choke coil 13, as disclosed in application Serial No. 164,413, filed January 29, 1927, which matured into U. S. Patent No. 1,687,225 on October 9, 1928, to which reference may be made for a more complete description.

In either case, by proper design,the direct current resistance of the choke coil maybe made small compared to the internal grid-cathode'resistance, so that when grid current flows the grid bias will remain'relatively unaffected by the potential drop across the choke coil.

Fig. 2 shows a modulating system employing two space discharge devices 15 and 16 connected in push-pull relation between an input transformer 17 and an output transformer 18.

Waves from a source 19, which may be a carrier generator, are supplied to a common input lead 30 through a transformer 20. Modulating waves may be applied to individual input circuits 3130 and 32-80 through input transformer 1'7. Grid current modulation is effected in the input circuits of devices 15 and. 16.

windings 21 and 22 of a choke coil are connected in series with the secondary winding of the input transformer 17 to increase the impedance of the individual input circuits to the frequency of the desired 'modulation product. Windings 21 and 22 are in parallel opposing relation for waves supplied through the secondary winding of transformer 20. By careful matching, the parallel-opposing impedance of the windings may be made small or negligible in comparison with the impedance of the remainder of the common input circuit.

If the frequency of the wave induced in the secondary winding of transformer 1'7 is high compared with the frequency of the modulation product, then windings 21 and 22 may be designed to be resonant at a frequency low compared with the frequency of the waves supplied by source 1.

The impedance of windings 21 and 22 in series aiding relation will then be negligible to the induced waves, but relatively high to the modulation product.

If the frequency of the wave induced in the secondary winding of transformer 1'? is low compared with the frequency of the modulation product then the portion of the circuit of Fig. 2 appearing to the left of the line X, X may be replaced by a circuit such as that shown in Fig. 2c.

Referring to Fig. 2-00, the transformer 17 may be designed for efficient transmission of the modulating wave. If such wave is of low or audio frequency the transformer may have an iron core. The windings 21 and 22 may be designed to be resonant at a frequency relatively high compared with the frequency of the modulating wave induced in transformer 17. The impedance of windings 21 and 22 in series aiding relation will then be negligible for waves supplied through 17, but relatively high to the modulation product.

In the common input iype of push-pull circuit, where both carrier and modulating waves are applied through input transformer 17, a similar result may be obtained by using a choke coil in the common input lead.

The above described methods of adjusting the impedance of the input circuit to increase the efficiency of a grid current modulation system are not possible if a non-inductive resistance is employed as the means for causing the potential to divide differentially between the discharge device and the'external circuit. The feasibility of frequency selective control constitutes an advantage inherent in. the use ofa reactive impedance arrangement. A choke coil is especially adaptable to the purpose, because it combines in a single circuit element a high impedance to the modulation product, a low impedance to the applied waves, and a low direct current resistance or grid leak.

What is claimed is:

1. A combined modulator and amplifier comprising a space discharge device having an input circuit adapted to combine impressed waves and generate modulation products therefrom, and having an output circuit cooperating with said input circuit to effect amplification, two sources of waves to be combined, said sources being con nected to the input circuit of the space discharge device, and an inductance coil in the input circuit, said coil being resonant at a frequency in the neighborhood 'of the frequency of a desired modulation product, whereby the coil has a rela-, tively high impedance to said modulation product and a low impedance to waves of at least one of said impressed frequencies, whereby the space discharge tube is efliciently used both as a modulator and as an amplifier of the desired modulation product.

2. A combined modulator and amplifier comprising a space discharge device having an input circuit adapted to combine impressed waves and generate modulation products therefrom, and having an output circuit cooperating with said input circuit to effect amplification, two sources of waves to be combined, said sources being connected to the input circuit of the space discharge device and an inductance coil in the input circuit, said coil being resonant at a frequency in the neighborhoodof the frequency of a desired modulation product, whereby the coil has a relatively high impedance to said modulation product and a low impedance to waves of at least one of said impressed frequencies, and said coil having two sections connected in parallel opposing relation to the source of the second impressed fre': quency to provide a low impedance for said second frequency, whereby the space discharge tube is efficientlyused both as a modulator and as an amplifier of the desired modulation product.

3. A combined modulator and amplifier having a space discharge device having an input circuit adapted to effect modulation and an output circuit cooperating with said input circuit to effect amplification, two sources of waves to be combined, said waves respectively having frequencies differing from each other by an amount materially smaller than'the lower of the two, said sources being connected to the input circuit of the space discharge device to generate a modulaiion product having the difference frequency, and an inductance coil in the input circuit, said coil having a natural resonance at a frequency below the lower impressed frequency, whereby the input circuit is made to have a relatively high impedance to waves of the difference frequency and a low impedance to waves of both impressed frequencies and the space discharge de- Vice is efficiently used both as a modulator and as an amplifier of the difference frequency Waves.

4. A modulating system comprising a plurality of space discharge devices each having a cathode, a grid and an anode, a divided inputcircuit having an individual portion connected to each of said grids, and a common portion connected to said cathodes, means for supplying a wave to said individual portions, means for supplying a wave of differentcharacteristics to said common portion, and an inductance coil having sections connected in series aiding relation with respect to said individual portions and resonant in the neighborhood of the frequency of a modulation product of said impressed waves to give said individual portions an impedance high compared with the grid cathode impedance of said devices at the frequency of said modulation product, said coil being adapted to have a negligibly small reactance at the frequency of the wave supplied to the individual portions of the input circuit by virtue of being out of resonance with said wave, and said coil sections being balanced so as to have negligibly small inductance for the wave supplied to the common portion of the input circuit.

EUGENE PETERSON. 

